During manufacture ceramic articles are fired at high temperatures and vitreous ware, for example, may be fired at temperatures in the range of 1000.degree. C. to 1450.degree. C. During the firing operation it is necessary to provide a support or supports for the articles and different types of known supports include refractory setters, batts, props and saggars. A setter for example is used to support and protect the shape of the ware from deformation and sagging as it passes through its vitreous, plastic stage during the heat treatment.
Conventionally, setters and other kiln furniture have been made from refractory materials such as cordierite--a magnesium, aluminium and silicate material--(for temperatures up to about 1260.degree. C.) and silicon carbide (for temperatures higher than 1260.degree. C.). These materials are relatively dense and have high thermal mass. Therefore the firing cycle for the desired ceramic products may be governed more by the properties of the supports than those of the precursers of the desired products.
More recent proposals have attempted to overcome this problem by the incorporation of ceramic fibers into the compositions from which the refractory supports are made, the use of the fibers with a suitable binder and filler material enabling the density of the products to be reduced while also giving an improvement in thermal mass properties.
However, these proposals have not been entirely satisfactory in practice and a need still remains for a light, low thermal mass product that can be used as a refractory support without causing problems such as "plucking". Plucking is a well-known phenomenon in this field and is caused by adhesion between the ceramic article being fired and its refractory support. Removal of the article from the support results in portions of the support being detached and sticking to the article, causing defects and rejects. In order to avoid this problem it has been found necessary to apply a surface coating of e.g. zircon or alumina, over the refractory support.
We have now found that this problem of plucking can be avoided, whilst retaining the low density and thermal mass of the desired support, and maintaining the required strength of the support, by a particular combination of binders in the composition and that the need for a provision of a final separate surface coating step can be eliminated.